1. Field of the Invention
The present invention relates to an operational amplifier (op amp), and more particularly, to an operational amplifier circuit having a digitally controllable output stage.
2. Description of the Prior Art
The operational amplifier (op amp) is an essential circuit building block of universal importance. A reason for the popularity of the op amp is its versatility; a great amount of operations can be done with op amps. Normally, an operational amplifier circuit is used to drive a load according to an analog input signal. However, if a digital input signal is adopted, a digital-to-analog converter (DAC) must be set in the signal input end of the operational amplifier circuit to convert the digital input signal into an analog input signal. The operational amplifier circuit can then drive the load according to the analog input signal.
For instance, the liquid crystal display (LCD) source driver is a kind of device that needs to utilize an operational amplifier circuit including a DAC to drive a LCD display unit according to a digital input signal. FIG. 1 shows a schematic diagram of a conventional operational amplifier circuit 100 utilized in a LCD source driver. The operational amplifier circuit 100 shown in FIG. 1 includes a DAC 110, an analog control stage 120, and an output stage (which comprises five transistors T1˜T5). The LCD display unit driven by the operational amplifier circuit 100 is represented by a block “load 190” in FIG. 1.
The DAC 110 converts a digital input signal SIN of the LCD source driver into an analog input signal S′IN. The analog control stage 120 then generates a control signal VC according to the analog input signal S′IN. With the five transistors T1˜T5 of the output stage, currents are sourced to and sank from the load 190 according to the control signal VC, the load 190 is therefore driven by the operational amplifier circuit 100 according to the digital input signal SIN.
In the conventional operational amplifier circuit 100, gates of the second and fourth transistors T2 and T4 are coupled to a constant bias voltage VBIAS, hence the two transistors can be thought of as constant current sinks for providing bias currents. After the operational amplifier circuit 100 is properly installed and supply voltages are provided to the operational amplifier circuit 100, power will be continually consumed by the second and fourth transistors T2 and T4. In practice, the operational amplifier circuit 100 is not always under operation, even if supply voltages are provided to the operational amplifier circuit 100 properly. For example, if no digital code is inputted into the operational amplifier circuit 100 as the digital input signal SIN, the operational amplifier circuit 100 will not be under operation. However, when the operational amplifier circuit 100 is not under operation, power is still continually consumed by the second and fourth transistors T2 and T4. In other words, under the structure of the conventional operational amplifier circuit 100, some power is unnecessarily wasted.